1. Field of the Invention
The present invention relates to a process for the production of thin films having optical properties.
This process makes it possible to produce thin films having e.g. antireflection, hydrophobic or abrasion resistance properties. These thin films can also have the properties of a dielectric mirror, which reflects one or more specific wavelengths, whilst having a relatively low intrinsic absorption.
2. Description of the Prior Art
The thin films relate to an organic or inorganic substrate (particularly plastics or vitreous substrates), coated with a layer having the sought optical properties. These thin films have numerous applications, particularly in the following fields: high power lasers, solar, thermal and photovoltaic applications, integrated optical systems or in architectural applications such as external glazed panels. In the field of solar applications, said films are used in optical systems for minimizing heat losses, concentrating and focusing light energy and finally protecting certain absorbent elements. The great interest awakened by such thin films with optical properties has resulted in the development of various manufacturing processes. Apart from methods making it possible to deposit optical films on various substrates, namely vacuum evaporation, reactive plasma and fluorination processes, which are costly and lead to heat treatments at high temperatures, the prior art discloses chemical processes such as sol-gel deposition processes. This type of process makes it possible to produce films placed on substrates and having various optical properties without requiring a thermal stage at high temperatures.
Among the sol-gel deposition processes, one of these methods consists of preparing colloidal treatment solutions and the deposition thereof on a substrate. In other words this method consists of forming a stable homogeneous suspension of solid particles (colloids) in a liquid solvent, said suspension constituting what is called a "sol" and then allowing said solvent to evaporate. For producing thin films, the solvent used must be sufficiently volatile to evaporate easily and provide space for a deposit of solid particles on the substrate. The prepared sol is generally deposited on the substrate by dip coating, spin coating, spray coating, slip casting or tape casting.
Examples of thin films obtained by the sol gel method are e.g. described in U.S. Pat. Nos. 4,929,278, 4,966,812, or U.S. Pat. Nos. 2,432,483 and 4,271,210.
Moreover, deposition processes for such colloidal coatings have also been described. Thus, an Article entitled "Colloidal Sol-Gel Optical Coatings" was published in the American Ceramic Society Bulletin, vol.69, no.7, pp. 1141-1143, 1990, and describes the possibility of depositing several films of colloidal materials by spin coating, in order to produce the optical components of a laser. This Article states that by using sol-gel colloidal suspensions and by appropriately choosing the volatile solvents constituting the liquid phase of the colloidal medium, it is possible to carry out treatments at ambient temperature without excessive heating of the substrate.
However, the spin coating deposition process suffers from a certain number of disadvantages. For example, the size of the substrates is limited to small dimensions and the corners of square or rectangular substrates are not correctly coated with a uniform film of the substance which it is wished to deposit there.
The other deposition processes also suffer from a certain number of disadvantages.
Dip coating requires the preparation of large quantities of solution for immersing the substrate to be treated. This is not very advantageous in the case of multilayer coatings for large-size optics. The conventional vacuum deposition methods (PVD, CVD) makes it possible to obtain high quality deposits, but for this purpose involve a difficult and expensive procedure (use of a vaporization bell).
In addition, German patent application DE 39 39 501 discloses a laminar coating device making it possible to make deposits of films on planar substrates. It is thus possible to produce photographic coatings, as well as electronic coatings (flat screens).
Finally, U.S. Pat. No. 4,370,356 discloses a deposition process using a hollow cylinder or roll filled with the substance to be deposited and which is displaced in translation relative to a substrate, so as to deposit on the surface thereof a thin film of said substance. This process is more particularly intended for the deposition of photosensitive resins, lacquers, antireflection coatings or polyimides.